This invention relates to a lens system for improving the field of view of the visually handicapped and more particularly to an anamorphic lens system.
There are patients who exhibit visual handicaps that manifest themselves in eye conditions that produce tunnel vision. Such patients usually have good central vision (within the narrow confines of the tunnel vision) and are limited to fields of view of about 5 to 15 degrees. Accordingly, such persons virtually have no peripheral vision. Such a condition which manifests itself in tunnel vision is found in cases of Retinitis Pigmentosa.
Retinitis Pigmentosa is an inherited disease which is usually first observed in patients between the ages of 8 to 14 years. As these patients become older, the peripheral field of view. generally grows smaller and smaller. Although Retinitis Pigmentosa is a major cause of tunnel vision, there are other diseases that also produce a loss of the peripheral field. In any event, the afflicted person who may have a typical central field of 5, 10, or 15 degrees is still able to walk around and to perform other tasks by combining their eye movement with head movement to enable them to scan the surrounding environment. However, at any instant, they only see a very limited field and must mentally combine these many limited fields to perceive some semblance of real space.
Essentially, the ability of the brain to correlate such information enables them to function with rather limited field of view.
For normal eyes the average field of vision is about 135 degrees in each eye and binocularly such persons have a 180 degree field of vision. This, of course, is inherent with peripheral vision.
In contrast patients afflicted with Retinitis Pigmentosa may have a field of view as little as 5 degrees and hence must constantly scan the scene in front of them by eye and head movement to avoid bumping into people or objects.
The condition of tunnel vision as manifested by many of these afflictions has been known since ancient times and all attempts proposed to provide a practical solution to increase this field have failed for one reason or another.
A particular approach which was experimented with in the prior art was to prescribe a reverse telescopic spectacle to increase the field of vision. This approach has not worked for two main reasons.
First, the regular reverse telescopic spectacle does not minify space and allows the patient to take in a larger field of view, but at the same time, it also reduces the patient's visual acuity. That is, a reverse telescope will provide minification in both the vertical and horizontal meridians or planes and therefore reduces such objects in size, thus reduces his amount of vision or visual acuity.
For example, if the reverse telescope minifies by a factor of -2.0X, then the percentage of vision or visual acuity will be reduced by two times or cut in half.
The second objection results because of the reduction in size of the image, the patient experiences a misjudgement of position because the object in space appears to be twice as far away in the above example.
These two factors are inherent characteristics of all reversed telescopic systems which normally employ spherical lenses.
Basically, it is an object of this invention to maintain one meridian at a constant size by the use of a special telescopic spectacle. It has been determined experimentally that in offering such a system, the acuity of the patient is preserved while further allowing the patient to make accurate judgements in distances. By use of this invention the patient may walk up steps, work in a factory or at home and in general will be able to perform normal tasks with good visual acuity and distance judgement.
In describing the invention, it is designated as an anamorphic lens system and constitutes lenses having a different power vertically and a different power horizontally. In this manner the lens system produces minification in the horizontal meridian only, while leaving the vertical meridian with zero effect.
This minification of the horizontal meridian only provides the patient with a wider field of view by fitting, for example, 10 degrees of real space into the patient's own 5 degree field. Thus, the horizontal field of view for the patient is increased by the same percentage as the power of the anamorphic system, according to the minification, as will be explained.
The anamorphic lens assembly is essentially a reverse cylindrical telescope that minifies in one meridian or axis as the horizontal, while maintaining the image of the vertical meridian at the same size.